The present invention relates to the art of electric arc welding and more particularly to an improved system and method for controlling the power supply during a welding process.
In electric arc welding it is common to employ a high speed switching type power supply, such as an inverter or chopper, so the output current of the power supply can be adjusted by a high speed control loop involving current feedback. In such electric arc welders, there is an outer control loop for creating the current command in accordance with the comparison of the average arc voltage with a reference voltage. In some instances, the current command signal to the power supply is directly controlled by the outer control loop. Consequently, the inner high speed control is based upon current feedback and is adjusted to maintain an average voltage. These inner high speed control loops operate at somewhat close to the speed of the inverter or chopper. The outer voltage control loop operates at about one-tenth the speed of the high speed switching power supply. Furthermore, the outer loop operates without the knowledge of the actual current used in the high speed inner control loop. Consequently, at low current operations, the arc current can dip too low and may cause extinguishing of the arc. Welding of aluminum GMAW requires more accurate control to maintain high weld rates. Thus, controlling the output current of a high speed switching power supply often involves a current overshoot because the outer control loop has a relatively slow reaction time. In short circuit and pulse welding where the feedback is average voltage, a complete weld cycle or portion of a weld cycle must be processed causing a relatively slow response time in the outer control loop. The control loop must be slowed down to maintain an average voltage to adequately compensate for long durations near zero voltage in a short circuit arc welding process. The control loop is thus very slow and the arc length is allowed to vary drastically during the welding process. With such a slow control loop the current is forced to transition by a greater amount to drive the arc length to the desired control voltage set by the outer loop. This presents two problem areas. In short arc welding at low currents, the background current is already so low that swings in the current, as experienced in a constant voltage system, forces the current too low and allow the arc to extinguish. This is especially noticeable just after each short is cleared. As the current is reduced to the background level, the control voltage system may overshoot and force the actual current to be driven too low allowing the arc plasma to be extinguished. This phenomena may occur during each weld cycle of a welding process so that intermittent extinguishing of the arc can be experienced unless corrective, expensive procedures are in place. Thus, welding at low currents is normally quite difficult when operating in short circuiting mode. When welding aluminum, fast changes in stick out or arc length caused by slow control cause intermittent disruption of the welding process with the resultant lack of uniformity of the weld bead.
The problems experienced in using a slow outer control loop with or without a high speed inner current control loop, as explained above, have been overcome by the present invention which essentially uses only the inner high speed loop for controlling the welding process. This is made possible by utilizing both the sensed arc current and the sensed arc voltage to produce a feedback signal indicative of the current and voltage product, i.e. power. Essentially, the electric arc welding process, whether it is spray, pulse or AC, has the actual arc power controlled by the desired power. To accomplish this result, both the sensed current and the sensed voltage are used. In the past, the arc voltage controlled the outside loop with its slow response time. The inner high speed loop was merely a current responsive feedback system where a digital error amplifier adjusted the input to the pulse width modulator driving the high speed switching power supply. By utilizing the product of the actual output current and voltage feedback signals, the actual output watts or power is sensed. This signal P is compared to the desired power established by the wave shaper. This causes the output current to change in a manner that forces the actual power to equal the desired power. This is performed at a speed generally ten times as fast as an outer loop control. By using a feedback signal based upon a relationship between both the current and voltage, the arc length is automatically adjusted and the rapid response time prevents any current overshoot.
In accordance with the invention, the voltage and current product is multiplied by a factor k. This product kP is then introduced into the high speed controller for maintaining the arc power at a set amount correlated with the time position in a welding cycle. The slow outer loop control is no longer required. The welding current depends on both the voltage and current introduced into the high speed control loop of the welder. The preferred implementation involves a feedback signal comprising merely the product of arc voltage and arc current. This is the arc power P. This product may be modified by a multiplier k. This factor can be used to compensate for travel speed or wire feed speed. This feedback signal is kP, but k is normally 1.0.
In accordance with a further aspect of the invention, the power P of the arc is a feedback signal for comparison with a power signal or profile from a wave shaper. In this aspect, the wave shaper generates a power profile, especially during arcing conditions of the welding cycle. For instance, when a short circuit type welding is being performed, such as the STT welding process, the wave shaper provides a desired arc power to be created after a short has been broken and an arc is reestablished. When welding in a pulse weld process, the wave shaper outputs peak arc power and then background arc power for comparison with the power feedback signal kP in the high speed control loop. In an AC arc welding process, the desired arc power during the positive polarity is outputted by the wave shaper for comparison with the power feedback signal kP during the actual positive polarity. In a like manner, the desired power during the negative polarity is outputted by the wave shaper for comparison with the power feedback signal kP to control the arc power of the negative polarity. Of course, as used in aluminum welding, the power during the positive polarity can differ drastically from the power during the negative polarity. In summary, a power signal is provided and a power feedback signal kP is sensed. These two parameters are compared to control the current from the high speed switching power supply. Of course, a wave shaper is not necessary during spray or globular welding when a constant power signal Pset controls the inner control loop and is used for comparison with the power feedback signal kP from the arc. The present invention results in substantial advantages, primarily at low current welding.
The feedback referred to as the power feedback function or signal kP is a relationship of sensed arc voltage and sensed arc current, which product may be multiplied by a factor k that is a constant or a variable. In the preferred embodiment, the multiplier is a constant 1.0 so that the feedback signal is merely the arc power. This power feedback function or signal is compared to the desired arc power for control of the output current of the power supply. The multiplier k may be a constant, as explained, or a linear equation, non-linear equation, or some other equation. The factor k may depend on the actual voltage or current and is employed to modify the feedback signal in a manner to compensate for various welding parameters, such as wire feed speed, electrode travel speed, shielding gas, wire diameter, wire material, etc. It has been found that multiplier k can be current plus voltage divided by current times voltages to produce a slope of the type in a drooper machine used for stick welding or aluminum MIG welding. This factor k produces a straight line relationship between current and voltage to give a slope operating curve. As can be seen, multiplier k can have any value to produce a specific relationship between voltage and current. However, in practice, the feedback signal is arc power obtained by multiplying the arc current and the arc voltage. This arc power signal is compared with the arc power desired at any given time in the welding cycle. The advantage of controlling power to a set level is based upon the fact that the power can be regulated at the extremely high switching speed of the inverter or chopper. At this high speed, any changes in arc length will result in a change in the arc force that will either increase or decrease the arc length to maintain equilibrium. For instance, when the arc power is set to operate at a given level, such as 2,000 watts, equilibrium is established when the voltage is 20 volts and the current is 100 amperes. This produces the desired arc length. Should the arc length increase, the output power is remained at 2,000 watts. However, the voltage increases, for example such as to 22 volts. This causes the current to drop to a level, such as 91 amperes. The reduced current reduces the arc force and thus causes the arc length to be reduced. This then decreases the voltage back to 20 volts and the current increases to 100 amperes. In a like manner, should the arc length be reduced and become too short, the output power remains at a fixed 2,000 watts. Consequently, the voltage decreases. This causes an immediate high speed increase in the current. Increased current increases the arc force and, thus, tends to force a greater arc length. Consequently, the short arc length is increased and the current and voltage seek equilibrium to produce the 2,000 watt controlled power level. Arc length variations using the present invention are minimized. Rapid stability is maintained. By using the invention, the power supply quickly reacts to changes in arc length so that the current will swing only a minor amount to maintain the desired set arc length. This advantage is especially noticeable immediately after each short is clear. By using the invention, the controller quickly seeks equilibrium of arc length and maintains it, even after the abrupt clearing of a short.
As is well known in aluminum GMAW welding, arc length is more difficult to control due to the low resistivity and melting temperature of the wire. The rapid control of the arc length, as obtained by the present invention, allows the power supply to maintain consistent control over the arc length, even when welding aluminum. Consequently, even with fast changes in stickout, the arc length is consistently maintained.
The invention is applicable to a pulse welding system, as well as a welding process with a constant desired arc power. In a pulse welding implementation of the invention, high speed control of both the peak output power and the background output power is obtained. In the past, pulse welding involved adaptive controls, wherein the arc length was determined by an adaptive loop based upon changing frequency, or a combination of peak current, background current and frequency. Consequently, the feedback control loop operated only once per pulse, whereby calculations were made and corrections were implemented for the next pulse. The present invention overcomes this problem and merely controls the power during peak and the power during background. This is not adaptive. It is real time power control to obtain the advantage of maintaining constant arc length during peak and background portions of the pulse welding cycle. Pulse welding with the present invention causes self regulation of the arc length faster than obtained with conventional adaptive pulse welding controllers. By using power to control low current during the background portion of a pulse wave improves the robustness of the power supply making it more resistant to pop outs.
It has also been found that the present invention is applicable for mixed polarity welding, such as GMAW, FCAW, SAW, MCAW, SMAW and other systems. All of these welding processes can be regulated by the feedback signal kP. For instance, if a square wave AC welding process is desired, a wave shaper provides the desired arc power for the positive polarity and the desired arc power for the negative polarity. High speed regulation during these different power levels quickly controls the arc length during the positive and negative pulses. The magnitude of the positive power may be different than the magnitude of the negative power in accordance with standard welding practice. While the shape and time of the wave form may change, based upon other conditions, the control of the wave form is based upon a power profile, which power profile is compared with the actual power feedback signal kP from the arc to maintain the arc power in accordance with the desired profile. This power profile is outputted by either an analog or, preferably, a digital wave shaper. The wave shaper produces a desired power profile to control the arc power at all portions of the AC welding cycle.
In accordance with the basic aspect of the invention, the feedback signal for the high speed control loop is arc power. This is the product of the sensed arc voltage and the sensed arc current. To modify the feedback signal, the invention also envisions use of a multiplier referred to as factor or multiplier k. The factor k may be a variable amount between 0.5 and 1.0. As indicated before, 1.0 is preferred. As the voltage changes the current control signal to the power supply changes to maintain constant arc power. The use of the factor k provides flexibility to change the relationship between the current and voltage. The use of the factor k does not change the power feature of the feedback signal. Thus, when the feedback xe2x80x9cpowerxe2x80x9d signal is referred to in discussing the invention, it is normally the product of current and voltage, i.e. P,; however, it can have a factor multiplier used, i.e. kP. In reality the feedback signal can be referred to as kP with k=1.0 when the signal is actual arc power P. Thus, power is used in the invention and is defined as a control variable which is preferably arc power P expressed as kP. To produce a slope operating characteristic for the feedback signal as explained before, the factor k may be voltage plus current divided by voltage times current. Also, the relationship could be another linear kP such as avarc+bIarc+c, where a, b, and c are constants. When this k is multiplied by the product of voltage and current, the feedback signal is the arc voltage plus the arc current. This feedback signal is compared with the desired power signal to obtain the rapid response of the power supply during its switching operation as previously described. The factor k can vary based upon wire feed speed. For instance, the factor k could be 150/wfs. If the wire feed speed is 300 inches/minute, the factor k is 0.5. Thus, the wire feed speed can be used to change the factor k to modify the feedback signal. Factor k may also vary based upon gas or travel speed or the voltage itself. When the factor k is based upon time, the feedback signal is in joules. When voltage is the factor k, the feedback signal is IV2. Other possibilities for modifying the feedback signal involving the product of arc current and arc voltage product can be used, but they do not change the fact that the basic control feedback signal is a multiplication of the current and voltage at the arc. Factor k can be modified by several welding variables or combinations of such variables. Those variables now used are travel speed, wire feed speed, voltage, current, time, gas mix, actual electrode stick out, wire size or type, inductance setting or other settings.
In accordance with the present invention, there is provided a control system for an electric arc welder performing a welding process between an electrode and a workpiece. In the preferred embodiment, the electrode is a welding wire fed into the arc at a wire feed speed controlled by a motor operating the wire feeding drive. The system comprises a high speed switching type power supply, such as an inverter or chopper. This power supply has a switching frequency of at least 10 kHz by a controller with an input current control signal to adjust the output current of the power supply. The controller operates more rapidly than the switching sequence of the power supply, but control is at the switching speed. A first sensor senses the actual arc voltage, while a second sensor senses the actual arc current. A first circuit is then provided for creating a power signal representing the desired real time power level at progressive times during the welding process. In the preferred embodiment, the power signal is created by a wave shaper that outputs a desired power profile. When spray or globular welding is being practiced, the power supply is a fixed value of the desired arc power. To complete the control system, there is a second circuit for generating a function of the sensed actual voltage and the sensed actual current to give a function of these two arc parameters for controlling the arc power. Consequently, a third circuit is used for adjusting the current control signal to the power supply in accordance with the difference between the desired power signal at any given time and the function of the actual voltage and current. This function is a feedback signal kP that is compared with the desired power signal at any given time during an arc condition so that the arc power is maintained at the desired level. This results in the equilibrium of the arc length as described previously. If the welding process involves a cycle that has a short circuit, then the power supply is operated in a current feedback mode during the short circuit condition. Thereafter, the system shifts to control by the feedback function or signal kP during the successive arcing conditions in the welding cycle. The function or feedback signal kP is preferably merely the product of voltage and current with k=1.0. However, it can employ a non-unity multiplier, as defined above, which multiplier may be a fixed number or a variable number according to the desired modulation of the power feedback function used in the control system. The feedback power signal is preferably Iaxc3x97Va. In the context of a power signal kP, k=1.0.
In accordance with another aspect of the invention, all circuits are digital circuits and performed by a digital signal processor, in accordance with standard welding control technology. However some aspects of the control system may be analog circuits or analog components without departing from the intended spirit and scope of the invention. The high speed switching type power supply is preferably an inverter operated at a frequency greater than 10 kHz and, preferably, substantially greater than 18 kHz. Of course, a chopper uses high speed switching and has a high speed inner control loop that can be controlled by power feedback signal kP. Preferably, the first digital circuit is a wave shaper that outputs a power profile matching the desired power level at all times during a welding cycle.
The invention is used with various welding processes. If the weld process has a fixed power, the invention does not require a wave shaper. Variable power cycles, such as pulse welding and AC welding, are controlled by the feedback signal kP and employ a device to output the desired arc power. When implementing a short circuit welding process, current control is used during the short circuit condition and feedback signal kP is used during the arc condition. In the present invention, the comparison of the actual arc power with the desired arc power is accomplished by a digital error amplifier having an output error signal that controls the pulse width modulator for adjusting the output current of the high switching speed power supply. Other implementations are within the skill of the art.
In accordance with another aspect of the invention, signal or function kP is used as a feedback signal to adjust the wire feed speed. In this manner, the arc length is changed to regulate the desired power of the arc by a change in wire feed speed (WFS). In accordance with still a further aspect of the present invention there is provided a method for controlling an electric arc welder by sensing the actual output current, sensing the actual output voltage, creating a power signal, creating a real time feedback signal kP representing the power at the arc, and then adjusting the input signal to the power supply by a comparison of the power signal and the real time arc signal kP. In this manner, as the voltage decreases the current increases and vice versa. This method can be practiced by a variety of analog and digital control circuits implemented in the high speed control loop of the welder.
In accordance with another aspect of the present invention power feedback signal P or kP is modified by a value representative of a parameter, such as wire feed speed or electrode travel speed.
The primary object of the present invention is provision of a system and method for controlling a high speed switching power supply of an electric arc welder in a manner to maintain a desired arc power by using a power feedback function or signal kP. As a broader object, there is creation of the feedback signal kP for controlling a weld parameter of an arc welding process.
Yet another object of the present invention is the provision of a system and method, as defined above, which system and method causes arc length stability and reduces current overshoots, especially when operating at low current and when welding aluminum.
Still a further object of the present invention is the provision of a system and method, as defined above, which system and method is easily implemented with standard welding technology without drastically modifying the control system of the electric arc welder.
These and other objects and advantages will become apparent from the following description taken together with the accompanying drawings